Agents were questioning at least five witnesses, including Hoboken Mayor Dawn Zimmer's chief of staff and communications director, three sources with direct knowledge of the probe told NBC News.

Zimmer told federal prosecutors those witnesses could confirm she previously told them about an alleged conversation with Christie's Lt. Gov. Kim Guadagno, in which Guadagno connected Sandy relief funds to approving a development project in Hoboken.

Juan Melli, Zimmer's communications director, and Dan Bryan, her chief of staff, cited a request from the U.S. attorney's office that city officials not discuss the probe publicly in declining to comment to NBC News.

"I can confirm that the mayor has said that the U.S. attorney's office is in Hoboken and talking to people," Melli said.

A Hoboken city councilman, David Mello, told NBC News that Zimmer also told him last summer about the alleged conversation with Guadagno. He is arranging to give that account to the U.S. attorney's office, he said.

"I distinctly remember [Zimmer] saying that the lieutenant governor said, 'If this came out, she would deny it,'" Mello said. "I thought it was absurd and outrageous."

Guadagno said Monday that she was surprised and offended by the allegations, while strongly denying she suggested Sandy funds were tied to Zimmer's approval of the project.